In recent years, an optical communication system employing a wavelength division multiplex method is widely adopted according to a demand of increasing a capacity of communication data and increasing a communication speed. In the following, a communication system employing the method may be referred to as a “wavelength multiplex system”, and an optical signal to be transmitted and received by the method may be referred to as a “wavelength multiplexed signal”.
In the optical communication system, measuring an Optical Signal to Noise Ratio (OSNR) which is one of indexes representing transmission path characteristic of a communication path in operation is important from an aspect of maintaining and managing communication quality.
Regarding such optical communication system, for example, the following techniques are disclosed. PTL 1 discloses a technique of verifying transmission quality of a new route to be set in an optical transmission network. The technique disclosed in PTL 1 extracts a transmission parameter representing transmission characteristics of a new route, based on a result of transmitting and receiving a test signal by using the new route, and verifies transmission quality, based on the transmission parameter.
PTL 2 discloses a technique of measuring an OSNR of a channel for OSNR measurement by lowering a bit rate of an optical signal of the channel. The technique narrows a spectral width of an optical signal by lowering a bit rate of an optical signal of a channel to be measured, and measures a noise component of the channel to be measured in a state that an influence due to an adjacent channel is reduced.
PTL 3 discloses a technique of measuring (estimating) an OSNR without using a power monitor for measuring a signal intensity of each channel in an optical repeater constituting an optical transmission network. The technique disclosed in PTL 3 calculates a gain tilt of each channel in an optical repeater from a total optical power of the optical repeater, and obtains the OSNR of each channel in the optical repeater by using the gain tilt.